Study On The Effect Of Timing Of Second Lining Construction On The Safety Of Tuff Tunnel

The tunnel lining plays a important role in the stability of the surrounding rock.Tunnel structures are in the hidden environment and the geological conditions are extremely complicated.Due to construction errors,unreasonable design,and inaccurate surveys,most of the tunnels have not reached the end of their service life,there are diseases affecting the safety of the secondary lining structure,The secondary lining will seriously affect the safety and durability of the tunnel.Tuff tunnels have a large deformation rate,a long duration,and are difficult to stabilize before the second liner applied.Basically can not meet the specification requirements of the deformation rate index,The second lining must be applied in due course so that the second lining bears the load.In this paper,through the investigation and study of the geological conditions of the project area,combined with the indoor and outdoor tests,Analysis of the characteristics of tuff in the project areathe.Through the monitoring and numerical simulation of the construction period,the deformation characteristics,initial and secondary lining load characteristics of the surrounding rock structures with different lining support timings were analyzed.Through the results of monitoring measurement and numerical simulation,the impact of the timing of the second line support on the structural safety is analyzed,and the safety factor of the second liner structure under different timing supports is compared to provide a reasonable support timing.The main research results are as follows:1.The mineral composition analysis test,physical property test,and mechanical test of tuff through indoor and outdoor test,the characteristics of plasticity,swelling,rheology,and damage deterioration are obtained.2.The characteristics of the surrounding rock deformation were studied through on-site monitoring,and the stress change law of lining structures under different timing of lining construction was analyzed.The sections under lining were monitored under four working conditions at a distance of 1 to 4 times of the tunnel diameter,and the displacement law of the surrounding rock and the stress variation of the second liner and the initial branch under different conditions were analyzed.Premature lining construction will lead to too much stress on the second lining,which is not conducive to structural stability.Failure to timely construct liningv may cause large deformation or even damage to the initial branch.The optimal support timing is obtained by studying the law.It is concluded that the effect of the second lining is optimal when it is 3 times the distance of tunnel diameter.At this time,both the lining and the initial support fully exert their bearing function,and the displacement of the surrounding rock is controlled without affecting the construction of the second lining support.3.According to the distribution rules of the primary branch and the secondary lining stress of the tunnel,it can be known that the timing of the secondary lining has a great influence on its stress distribution.The farther the distance from the construction surface,the better for the safety of the secondary lining structure.Lining has not been followed up in a timely manner,and the stress on the initial branch has continued to increase.This will cause cracking and spalling of the initial support concrete,deformation of the steel arch,and excessive displacement of the surrounding rock will affect the construction of the second lining.4.Through numerical simulation,the displacement of the surrounding rock of the tunnel in 4 kinds of working conditions,changes in the plastic zone,and the value of the axial force of the second liner are analyzed.The change law of the safety factor of the second liner of the tunnel during the construction period and after one year is studied in combination with the creep phenomenon to obtain the optimal support timing.The creep of soft rock has a long duration.After 1 year,the axial force of the surrounding rock changes significantly.The earlier the support,the longer the axial force of the second lining increases,and the second lining works when the distance from the face is 1 and 2 times of the diameter.Its safety factor is far below the 2.4 which standard requirement.It was found that the second lining was applied under the condition of three times the diameter of the face,and the structural safety factor was continuously stable above 2.4.